Glaucoma is an eye condition typically characterized by an increase in the intraocular pressure (IOP) of the eye to an abnormal level. A normal eye maintains a proper IOP by the circulation within the eye of aqueous humor. Aqueous humor is secreted from the ciliary body, passes through the pupil into the anterior chamber of the eyeball, and is filtered out of the eyeball via the trabeculum and the Canal of Schlemm (or Schlemm's Canal). With glaucoma, the aqueous humor excretory pathway is blocked, the aqueous humor cannot pass out of the eyeball at an adequate rate, the IOP rises, the eyeball becomes harder, and the optic nerve atrophies by the pressure applied on its fibers leaving the retina. A characteristic optic neuropathy develops, resulting in progressive death of the ganglion cells in the retina, restriction of the visual field, and eventual blindness. Advanced stages of the disease are characterized also by significant pain.
Glaucoma treatment, if initiated early in the course of the disease, can prevent further deterioration and preserve most of the ocular functions. The goal of glaucoma treatment is to reduce the IOP to a level which is considered safe for a particular eye, but which is not so low as to cause ocular malfunction or retinal complications.
In the past, procedures and devices have been developed and implemented for providing an alternate route for aqueous humor to pass out of the eye. For example, in full thickness filtration surgery, a fistula is created through the limbal sclera, connecting directly the anterior chamber of the eyeball and the sub-conjunctival space. This provides an alternate route allowing the aqueous humor to exit the anterior chamber of the eyeball through the fistula in the limbal sclera and to pass into the sub-conjunctival space. In guarded filtration surgery (trabeculectomy), a fistula created through the limbal sclera is protected by an overlying partial thickness sutured scleral flap. Again, this provides an alternate route allowing the aqueous humor to exit the anterior chamber of the eyeball, through the fistula in the limbal sclera, and allowing the aqueous humor to pass into the sub-conjunctival space.
Drainage implant devices have also been developed and implemented. For example, some implants have a tube that is inserted through the limbal sclera. The tube provides an alternate route for the aqueous humor to leave the eye.
Many of these known devices and methods do not provide adequate regulation of IOP. For example, with some devices and methods, the initial procedure can cause excessive loss of aqueous humor from the eyeball during the early postoperative period, frequently leading to hypotony. With other devices and methods, there may be too much resistance to the flow of aqueous humor from the eyeball, thereby resulting in higher eventual IOP and an increased risk of late failure. There is also the risk that the drainage pathway will become clogged due to scarring or that infection could occur because of the passageway into the eye. In certain valved implant devices, defects in and/or failure of the valve mechanisms can lead to either too much or too little aqueous humor exiting the eye. In procedures that drain into a “bleb” in the sub-conjunctival space, there is sometimes a risk of leakage or infection.
In order to provide improved IOP regulation, some alternative procedures and devices have been proposed to attempt to utilize the eye's natural physiological drainage pathway through the Schlemm's Canal. If the cause of the rise in IOP is a blockage of flow through the trabecular meshwork between the anterior chamber and the Schlemm's Canal, then a device or procedure that opens one or more passageways through the trabecular meshwork or provides a bypass route from the anterior chamber to the Schlemm's Canal can help reduce IOP. To date, none of the procedures or devices that have been proposed to enhance flow from the anterior chamber to the Schlemm's Canal has proven completely satisfactory.
In a deep sclerectomy, a superficial flap is made in the sclera and then a second deep scleral flap is created and excised leaving a scleral reservoir under the first flap. A thin permeable membrane is exposed between the anterior chamber and the scleral reservoir. The procedure is non-penetrating in that no penetration is made into the anterior chamber. The aqueous humor percolates from the anterior chamber through the thin membrane into the scleral reservoir and into the Schlemm's Canal. This procedure can be difficult to perform and has not been shown to be fully effective in reducing IOP.
Trabeculoplasty procedures are a group of procedures where a physician uses a laser to create holes in the trabecular meshwork to allow flow from the anterior chamber into the Schlemm's Canal. The two primary types of trabeculoplasty are argon laser trabeculoplasty (ALT) and selective laser trabeculoplasty (SLT). Trabeculoplasty may not be a suitable long-term treatment as the meshwork may close again, for example due to scarring.
The TRABECTOME™ device of NeoMedix, Inc., has been proposed as another method for providing passage through the trabecular meshwork. The device is passed through a corneal incision and across the anterior chamber. The device's tip has a bipolar micro-electrocautery electrode that ablates and removes a strip of trabecular meshwork. As with trabeculoplasty, this procedure may not be a suitable long-term treatment as the meshwork may close again.
The viscocanalostomy procedure uses a viscoelastic material in a procedure similar to the deep sclerectomy procedure. The physician injects a viscoelastic material, such as sodium hyaluronate, into the Schlemm's Canal from the scleral reservoir. The viscoelastic material opens the Schlemm's Canal and helps to insure the patency of the passage from the scleral reservoir to the Schlemm's Canal. The viscoelastic material is claimed to increase the permeability into the Schlemm's Canal and to help prevent closure of the passage due to fibrongen migration and scarring. Like the deep sclerectomy procedure, the viscocanalostomy procedure can be difficult to perform and has not been proven to be fully effective in reducing IOP.
Canaloplasty is a procedure similar to viscocanalostomy with the primary difference being that viscocanalostomy attempts to open only portions of the Schlemm's Canal adjacent the scleral reservoir, while canaloplasty attempts to open the entire length of the Schlemm's Canal. In canaloplasty, a microcannula is inserted into the Schlemm's Canal at the sceral reservoir and passed all the way around the Schlemm's Canal, in conjunction with the injection of a viscoelastic material around the Schlemm's Canal. A suture is then tied to the microcannula, and the microcannula is withdrawn back around the Schlemm's Canal, pulling the suture through the Schlemm's Canal. The suture is tied together at its ends to apply pressure, stretching the trabecular meshwork inwards and helping open the Schlemm's Canal. Like viscocanalostomy, the canaloplasty procedure can be difficult to perform and has not been proven to be fully effective in reducing IOP.
Two devices that have been proposed to help flow into the Schlemm's Canal are the iStent device of Glaukos Corp. and the EyePass device of GMP Companies, Inc. The iStent device is inserted into the Schlemm's Canal by an ab-interno procedure, while the EyePass is inserted into the Schlemm's Canal by an ab-externo procedure.
The iStent device of Glaukos Corp. is a small L-shaped titanium tube that is implanted through the trabecular meshwork into the Schlemm's Canal. Multiple implants may be used around the circumference of the Schlemm's Canal. The iStent device does not appear to be fully effective in reducing IOP without the need for several implants.
The EyePass device of GMP Companies, Inc., is a small, generally Y-shaped silicon tube that is used in a procedure similar to deep sclerectomy, without the need for the creation of the thin membrane for percolation of aqueous humor from the anterior chamber to the scleral reservoir. The EyePass device is placed in the scleral reservoir with its inlet branch entering the anterior chamber and its two outlet branches passing into the Schlemm's Canal. The EyePass device does not appear to be fully effective in consistently reducing IOP.
None of the known devices or procedures provides a fully satisfactory solution in terms of consistently achieving optimal IOP in an efficient manner.